1. Field of the Invention
Exemplary embodiments of the present invention relate to an optical fiber manufacturing method and an optical fiber manufacturing apparatus for drawing an optical fiber from an optical fiber preform, thereby manufacturing the optical fiber.
2. Description of the Related Art
FIG. 5 is a schematic view illustrating a configuration of a conventional optical fiber manufacturing apparatus. In general, an optical fiber is manufactured by the following operations.
(1) An optical fiber preform 101 that is a glass rod used as a basis for an optical fiber is inserted into a heating furnace 102. The front end of the optical fiber preform 101 is heated and melted by a heater 102a of the heating furnace 102 at a temperature of about 2000° C. to form a bare optical fiber 103. The bare optical fiber 103 is pulled from the lower portion of the heating furnace 102.
(2) Below the heating furnace 102, a cooling unit 104 that extends longitudinally in a vertical direction is provided. Inside the cooling tube 104A, a cooling gas such as helium gas is supplied from a side portion 104a of the cooling tube 104A at the longitudinal center position. As shown by a flow direction 110 in FIG. 5, the cooling gas is supplied into the cooling tube 104A at the center position and then spreads upwards and downwards. The bare optical fiber 103 pulled out of the heating furnace 102 is cooled by the cooling gas to a temperature at which it can be coated.
(3) In order to form a protective coating layer for protecting the surface of the bare optical fiber 103, first, the cooled bare optical fiber 103 is passed through a coating unit 106. The coating unit 106 applies a coating resin (not shown) to the surface of the bare optical fiber 103. Thereafter, the bare optical fiber 103 is passed through a curing unit 108 so that the coating resin is subjected to thermal curing or UV curing, thereby forming an optical fiber 107. The protective coating layer (not shown) formed of the coating resin as described above generally has a two-layer structure that includes an inner layer, made of a material with a relatively low Young's modulus, and an outer layer made of a material with a relatively higher Young's modulus.
(4) After the optical fiber 107 is passed through the curing unit 108, the optical fiber 107 is fed out through a turn pulley 109 to be wound by a winder (not shown).
In order to increase productivity for optical fibers in an optical fiber manufacturing method, an increase in the size of the optical fiber preform and an increase in drawing speed have been desired. In order to increase the drawing speed without raising a fiber drawing tower (not shown), which receives the optical fiber manufacturing apparatus therein, a development to increase the cooling efficiency with the cooling unit 104 used in the cooling operation of the bare optical fiber 103 described above in (2) has been made. Here, the cooling unit 104 generally uses helium gas having high thermal conductivity as the cooling gas. However, since helium gas is expensive, a development to reduce the amount of helium gas and increase the cooling efficiency has been made.
An example of a cooling method that reduces the amount of helium gas while maintaining an acceptable cooling efficiency is disclosed in the Japanese Unexamined Patent Application, First Publication No. 2003-95689. FIG. 6 is a schematic view illustrating a configuration of an optical fiber manufacturing apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-95689 (in the figure, the same elements as described with reference to FIG. 5 are denoted by the same reference numerals, and a detailed description thereof is omitted).
The optical fiber manufacturing apparatus is provided with a mechanism for supplying sealing gas which prevents cooling gas such as helium gas from being diluted with air. A helium gas inlet port 204a is provided at a vertically low portion (that is, on the output side of the bare optical fiber 103) of the cooling tube 204A of the cooling unit 204. In addition, a sealing gas inlet port 204b is provided vertically below the helium gas inlet port 204a. With such a configuration, the cooling gas supplied through the helium gas inlet port 204a is discharged from the vertically upper portion (that is, on the input side of the bare optical fiber 103) of the cooling tube 204A as shown by a flow direction 210 in FIG. 6. In addition, the sealing gas such as carbon dioxide gas (CO2) supplied through the sealing gas inlet port 204b flows toward the vertically lower portion of the cooling tube 204A as shown by a flow direction 211. Accordingly, the sealing gas is introduced from the sealing gas inlet port 204b disposed vertically below the helium gas inlet port 204a to cause the flow direction 211 of the sealing gas, thereby preventing the cooling gas from flowing out from the lower portion of the cooling tube 204A. Moreover, since the carbon dioxide gas is employed as the sealing gas, a dilution of the cooling gas (particularly the helium gas) lighter than air, with the sealing gas can be prevented. Accordingly, a decrease in cooling efficiency can be avoided, and the amount of the cooling gas can be reduced by 10 to 20% as compared with a conventional case.
As described above, in the apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-95689, the helium gas inlet port 204a is provided at the lower portion of the cooling tube 204A, and the sealing gas inlet port 204b is additionally provided below the helium gas inlet port 204a. Accordingly, it is designed so that the cooling gas flows upwards and the sealing gas flows downwards.
However, the practical flows of the cooling gas and the sealing gas are pulled by the movement of the bare optical fiber 103 or influenced by viscous resistance or the like that occurs when the gases flow inside the cooling tube 204A. For example, in some cases, due to the increase of a drawing speed from the start of the drawing until a product manufacturing condition, the change in the drawing speed during the drawing, and the like, the flows of the cooling gas and the sealing gas are not as desired. In this case, the cooling gas may be discharged from the lower portion of the cooling tube 204A, the flow of the cooling gas may become unstable, or the cooling gas may be mixed with the sealing gas to cause an unstable mixing ratio, thereby resulting in unstable cooling efficiency. Accordingly, there is a problem in that the apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-95689 has poor manufacturing stability and reproducibility.
Meanwhile, the thickness of the protective coating layer varies depending on the temperature of the cooled bare optical fiber 103. Therefore, in order to maintain the thickness (coating diameter) of the protective coating layer at a constant level over the entire length of the optical fiber 107, generally, the cooling ability of the cooling unit 204 is adjusted according to the change in the drawing speed.
The apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-95689 is provided with a mechanism for supplying cooling ability-adjusting gas to be mixed with the cooling gas. However, there is a concern that, in the apparatus, the cooling ability-adjusting gas and the sealing gas are mixed with each other, thereby resulting in a further unstable mixing ratio. In this case, problems such as degradation in response characteristics upon adjusting the cooling ability and in an adjustment of the cooling ability occur.
Exemplary embodiments of the present invention were devised in view of the above circumstances, and have an exemplary objective of providing an optical fiber manufacturing apparatus and an optical fiber manufacturing method which can reduce the amount of cooling gas and control a cooling ability with good response characteristics.